Radio transmission systems are required to further improve spectrum efficiency and realize high-speed data transmission. The OFDM (Orthogonal Frequency Division Multiplexing) scheme has been considered as one option of communication schemes meeting these demands.
Further, in radio transmission systems, support needs to be provided for various class of QoS (Quality of Service). In particular, support for interactive services such as game, telephone and video conference must realize extremely little communication latency, compared to file transfer and web browsing services.
Given this background, in systems using the OFDM scheme, consideration on the method of improving throughput is made by reducing the amount of pilot signals to estimate channel response and increasing the number of symbols and subcarriers to which user data is assigned.
For example, Patent Document 1 discloses setting the mapping pattern of pilot symbols for estimating channel response so as to be responsive to the variation in channel response according to the time variation and the frequency variation of the channel. In particular, as shown in FIG. 1, for users with respect to whom the time variation of the channel is moderate, the time interval for mapping pilot signals (the symbols illustrated with black symbols in the figure) is set wide, and, on the other hand, as shown in FIG. 2, for users with respect to whom the frequency variation of the channel is moderate, the subcarrier interval for mapping pilot signals is set wide (the symbols illustrated with black symbols in the figure), thereby improving transmission efficiency in either case. That is, as shown in FIG. 3, the pilot symbol mapping pattern for users with respect to whom the time variation and frequency variation of channel response are relatively moderate, is set as shown in FIG. 3.
When receiving processing is performed for the data symbols subject to receiving processing shown in FIG. 3 (shaded symbols in the figure), the channel response estimating method on the receiving side requires performing interpolation processing in the time domain and the frequency domain and estimating channel response at time tn, as explained below.
First, channel response estimation values of OFDM symbols between pilot symbols are calculated in subcarrier fk (k=1, 5, 9). That is, channel response estimation values hk (k=1, 5, 9) of subcarriers f1, f5 and f9 at time tn are calculated by estimating channel response estimation values hk(n−1) and hk(n+1) using pilot signals at time tn−1 and tn+1 and performing linear interpolation for estimated channel response estimation values hk(n−1) and hk(n+1).
Next, the channel response estimation values hk(n) (k=2, 4, 6, 8, 10) for the rest of the subcarriers are calculated, by frequency domain interpolation between channel response estimation values hk(n) (k=3, 7, 11) estimated by pilot signals at time to and channel response estimation values hk(n) (k=1, 5, 9) estimated by time domain interpolation.
As described above, interpolation processing in the time domain and the frequency domain makes it possible to find the channel response estimation value for each subcarrier at time tn, and data symbols subject to receiving processing are demodulated using calculated channel response estimation values.
Further, in the case of the pilot interval for moderate time variation of channel response shown in FIG. 1, by estimating channel response h(n) and h(n+1) using pilot signals at time tn and tn+1 and linear-interpolating estimated channel response h(n) and h(n+1), channel estimation values that are responsive to the time variation of channel response are found.    Patent Document 1: Japanese Patent Application Laid-Open 2004-530319